Toxic and carcinogenic impurities must be removed from their polymer product streams as completely as possible. Processes which can reduce such materials to minute levels are extremely desirable; moreover, inexpensive and simple methods for removing any impurity that is present in an amount of about 2% by weight of the polymer product stream, and reducing such an impurity to less than 0.5 parts per million (ppm) is particularly desirable.
A variety of methods have been developed in the art for the purpose of controlling impurity concentration in product streams. In producing alcohol ethoxylate for example, process controls are used to maintain a low level of ethylene oxide in the product stream. These operating procedures, however, reduce production rates for the alcohol ethoxylate. Moreover, in spite of process controls during production of the alcohol ethoxylate, ethylene oxide concentration levels can reach undesirable levels.
A known purification method is described in U.S. Pat. No. 3,000,963, relating to a process for refining crude polymers and adducts of propylene oxide. This patent describes a method of reducing impurities by neutralization, filtering, and then vacuum stripping. Another purification method is noted in U.S. Pat. No. 3,357,970, relating to ethoxylation of solid polyols. The purification method used in this reference is stripping with a nitrogen purge at 392.degree. F. under vacuum.
Spraying techniques have been used commercially for materials that are easy to separate. In the manufacture of alumina (Al.sub.2 O.sub.3), for example, solid alumina is separated from liquid by-product using a spray-drier. Other commercial processes applying spray techniques are used to separate materials such as solvents which are extremely volatile, and present in feed streams in large concentrations, (over 10% by weight of the total stream).
U.S. Pat. No. 2,346,783 teaches the use of spray nozzles which relates to the purification of ethers of diethylene glycol to increase the surface area between contacting phases. Purification is achieved via extraction, taking advantage of the higher solubility of the impurities in the contacting phase.
While known purification methods can succeed in removing more volatile impurities from a product stream, disadvantages inherent to such systems include expense, time consumption, low product output, production of other impurities, loss of desirable product, lack of efficiency and consumption of great amounts of energy.
It is also difficult, if not impossible, to significantly lower the concentration of impurities having vapor pressures near the vapor pressures of the feed stream material itself. Methods which heretofore have been used to separate volatile impurities from nonvolatile or solid materials are not suitable for the removal of impurities such as dioxane from feed streams such as alcohol alkoxylate.
It would be advantageous to provide a method that can remove impurities which are present in feed streams in small quantities. The present invention provides a method for removing impurities present in concentrations less than 2% by weight of the liquid feed stream from liquid polymer feed streams having maximum volatility levels to extremely low levels. This process is especially suited for impurities present in low concentrations. The present invention is particularly useful in dioxane removal from alcohol alkoxylates.